The invention relates to a Fe--Ni based alloy sheet having superior surface characteristic and superior etchability (i.e., superior etchability means that etching can be performed in a good state), which sheet is suitable for producing a shadow mask used in a display, a lead frame for an integrated circuit (IC) and etc.
Hitherto, a Fe--Ni based alloy sheet has been used as a raw material for electronic parts such as a shadow mask and an IC lead frame etc.
The Fe--Ni based alloy sheet for the electronic parts is subjected to, for example, such a fine photoetching working as to form electron-transmitting holes regarding the shadow mask and inner leads regarding the IC lead frame. However, in view of recent higher degree of precise-and-fine design and higher integration design, both of further enhancement of the etchability and further thinner design of the sheet have been required.
As means for improving the etchability, hitherto, reducing of the amount of Al.sub.2 O.sub.3 inclusions which is hardly etched and making the size of Al.sub.2 O.sub.3 finer have been tried, however, reducing of segregation of components such as Ni which segregation becomes the cause of the roughness of an etched surface is recently noted. As means for further achieving the thinner sheet design, it is noted to reduce the amount of the Al.sub.2 O.sub.3 inclusions.
In a conventional refining step for producing the Fe--Ni based alloy, Al having high deoxidizing ability has been used during the step of deoxidizing treatment as a main deoxidizer. Thus, in a resultant molten metal having been refined, Al.sub.2 O.sub.3 remains which occurs during the deoxidization performed by Al, with the result that Al.sub.2 O.sub.3 inclusions are confirmed to exist even in the structure of a Fe--Ni based alloy ingot. Since the Al.sub.2 O.sub.3 inclusions are high in melting point and in hardness, the Al.sub.2 O.sub.3 inclusions are hardly dissolved or dispersed even after forging and soaking treatment performed as to the Fe--Ni based alloy ingot, and the Al.sub.2 O.sub.3 inclusions are inferior in elongation during hot rolling and it is hardly impossible to make the Al.sub.2 O.sub.3 inclusions fine in size even after the Fe--Ni based alloy had been cold-rolled.
Thus, in a case of a sheet having a thickness of 0.25 mm, there occurs such a fear as the Al.sub.2 O.sub.3 inclusions are exposed on the surface thereof, which exposure becomes the cause of roll scratches and another cause of surface defects of the sheet and which exposure further becomes the cause of quality deterioration such as etching inferiority and distortion of a pattern configuration after the etching.
Further, in a case where it is intended to make a conventional Fe--Ni based alloy sheet have a thickness of 0.15 mm in order to achieve the further enhancement of characteristics, the occurrence of the surface flaw (surface defect) is more marked, that is, the thickness of 0.15 mm is a threshold value so far as the conventional Fe--Ni based alloy sheet is concerned.
In order to address the problems, for example, JP-A-7-252604 proposes a method of obtaining superior surface characteristic by limiting the amount of Al in Fe--Ni based alloy into a level of 0.02% and preferably not more than 0.01% so that the occurrence of Al.sub.2 O.sub.3 inclusions inferior in elongation during rolling may be suppressed.
As a method of decreasing Ni component segregation occurring in the Fe--Ni based alloy, for example, each of JP-A-60-128253 and JP-A-1-252725 proposes a method of applying soaking treatment to an ingot or slab.
The methods explained above are effective to reduce the surface flaws and the Ni segregation both occurring in the Fe--Ni based alloy. However, even in a case of adopting the method of JP-A-7-252604, it is impossible to sufficiently reduce Al.sub.2 O.sub.3 inclusions in the structure, and problems remain regarding the improvement in both of the surface characteristic and etchability of the Fe--Ni based alloy sheet. Further, even in a case of adopting the methods of JP-A-60-128253 and JP-A-1-252725, it is impossible to sufficiently reduce Ni segregation and a problem still remains regarding the improvement in etchability.